1. Field of the Invention
This invention relates to a method for forming images composed of dots in a matrix used for a printer, a light emitting diode (LED) printer, or in a bit map display unit such as a cathode ray tube (CRT), and more particularly to improve the images by smoothing stairsteps included in the dot images.
2. Description of the Prior Art
The prior art shows that a laser printer forms a picture element pattern or pixel pattern on a photosensitive drum along its coaxial scanning line in a matrix by scanning repeatedly laser beams modulated with video data sent serially from an image controller.
Other prior art teaches that a dot printer such as an LED printer forms a pixel pattern in a matrix on a recording member, a photosensitive drum for example, moving relatively vertically, by means for forming pixels such as an LED disposed along the coaxial scanning line, which is controlled to emit simultaneously in the full line or successively in block after block.
In either printer, the pixels are arrayed in an n.times.m matrix to form characters or pictures. Letters such as O and V consisting of curves or diagonals, therefore, are formed with stairstep edges which damage the quality of the picture.
To overcome the defects, in the laser printers which form pixel patterns by scanning the modulated laser beams along the scanning line, attempts for smoothing the stairsteps are disclosed to form a smaller dot size by reducing the output energy for the pixels at the diagonal edges (Laid Open Pat. Appln. 60-139072); or to modulate so as to narrow the width of the output laser beams and to print small dots within the normal dot area.
In the LED printers, on the other hand, arrayed with the diodes in a line, the diodes are disposed with a fixed distance along the scanning line. The LED's, further, are arranged in a modular tip with n units of diodes which results in control of the scanning line with the n units or the full line. It is, therefore in the LED printers, hard to adopt the above techniques which change the dot size and shift the smaller dots.
In an LED printer, therefore, which outputs a video image with a line unit, a technique for smoothing stairstep edges is constituted by a method for arranging pixels so that each pixel shifts in every scanning line within a suitable distance to dispose the pixels diagonally along the vertical scanning line (Pat. Publn. 62-24987).
In the prior technique, however, as the pixels shift in the full scanning line, it is hard not only to obtain a smoothed character, but also to absorb a steep stairstep even if the elements are arranged by logical adjunction.
To overcome the defects above, a technique is disclosed to replace the pixel in question with narrower pixels which are made coordinately narrower either along the horizontal scanning line or the vertical scanning line (Laid Open Pat. Appln. 2-112966).
In the prior technique, the replacement of the specific pixel in question is made on the basis of a reference pixel pattern having four compensation subcells which are narrower in a coordinate direction. The prior technique, in the LED printer, is able to provide narrower dots in the vertical direction by controlling the emitting time of the LED array. Because the LED is disposed in a fixed distance, the pixel has to be controlled the light intensity to make the pixel narrower in the scanning direction. As described above, because the LED's are combined in a tip with a plurality of diodes, it is substantially hard to control the light intensity of individual diodes.
U.S. Pat. No. 4,437,122 shows a 3.times.3 matrix of subpixels, referring the surrounding pixels. The technique is hard to adopt in LED printers, because an LED can not be divided along the scanning line. Further, the center pixel has to be referred with eight surrounding pixels, and to be matched with the 3.times.3 subpixel combinations, that is, 20 subpixel patterns including their rotation which results in preparing a sophisticated circuit, and in a prolonged processing time. Thus, the technique does not satisfy the needs for faster processing.
Each of the prior techniques consists of the steps of, firstly appointing the central pixel in question, secondly comparing the configuration of n.times.m neighboring pixels with many templates prepared in advance, and lastly smoothing the central pixel according to the template pattern matched. To compare the configuration of the neighboring pixels means that one has to prepare many templates corresponding to the combinations in the n.times.m matrix. Further, there is a meaningful stairstep to figure such corner as in letters E and F, which corner must be kept sharp. To achieve the effective smoothing and not to smooth the meaningful stairsteps, one has to prepare other templates for the stairsteps not to be subjected to the procedures which results in providing another memory for the meaningful stairsteps, and in constructing more sophisticated circuits. The large scale of the circuit and the large amount of work for matching delay the process and thus fail to satisfy the requirement for faster processing.